Encoding help, run into block.
 

Hey, im trying to make an assembly program which encodes a file using base64.

Anyway I have a program that works in converting the word "Man" to TWFu. Problem is it does not convert anything else but a few 3 letter words like Tan and Ban probally because of its similarity with Man. Below is a code snippet I am using to run the encode, taking 3 bytes from the read file and converting it to 4 bytes writing it to the file then grabbing another 3 bytes and doing the same thing. If anyone could help me figure out what I am doing wrong here help would be appreciated. I am completely stumped at this point and spent the weekend stuck on this one issue.

By the way this is x86 Assembly code, 16 bit. I am using Masm as my compiler.

Encode64:

        ;Read Input File (3 bytes)

        mov ah, 3fh 

        mov bx, input_handle 

        mov cx, Buff3b;seting the size to read at a time

        mov dx, offset input_pointer ;pointing the buffer where to store

        int 21h 

        jc End_Program



        ;Appends a 0 if the number of bytes read is less then 3

        ;ax = amount of read bytes

        cmp ax, 2

        jne start0

        mov input_pointer[2], 0

        start0:

                cmp ax, 1

                jne end0

                mov input_pointer[1],0

                mov input_pointer[2],0

        end0:

                or ax, ax

                jz done2 ;jump to done2 if ax=0

        

;;;;;;;;;;Convert the 3byte data into 4 byte data

;;;;;;;;;;Storing the output into a buffer for writing

        

        

        B1:

                mov al, input_pointer[0]

                shr al, 2

                add al,65d

                mov output_pointer[0],al



        B2:

                mov ax, word ptr input_pointer[0]

                xchg al, ah

                shl ax, 6

                shr ax, 10

                add al, 65d

                

                

                

                mov output_pointer[1], al



        B3:

                mov ax, word ptr input_pointer[1]

                xchg al, ah

                shl ax, 4

                shr ax, 10

                add al, 65d

                mov output_pointer[2],al

        B4:

                mov al, input_pointer[2]

                shl al, 4

                shr al,2

                add al, 61d

                mov output_pointer[3], al





;;;;;;;;;;;;;;;;;;;;;;;;; write 4b encoded data to output file 

                mov cx, buff4b;set number of bye to write

                mov ah, 40h 

                mov bx, output_handle 

                mov dx, offset output_pointer ;write from buffer

                int 21h 

                jc end_program

                jmp Encode64

Re: Encoding help, run into block.
 

I'm not sure exactly what problems you are having, but the stuff following B4 has some errors.

First, if you only want to clear the top two bits of AL then
shl al, 2
shr al, 2
Or, even simpler:
and al, 3Fh

Also, you only add 61 to it instead of 65 as you did in all the others.

You are aware, I presume, that simply adding 'A' doesn't account for three things: punctuation characters between A..Z and a..z, a control character (ASCII 127), and the fact that you exceed the ASCII character range by one character (your domain is 65..128, while ASCII is only defined on 0..127).

You would do better to use a lookup table for ASCII digits and use the xlat instruction to convert AL.

Hope this helps.

Re: Encoding help, run into block.
 

xlat is definately a nifty feature, just applied it. That said it is defiantely working better but still working hard on it!. I will look into more in depth how to fix it with the suggestions you have outline. I'll come back if I run into any road blocks but hopefulyl that wont happen.

Re: Encoding help, run into block.
 

Great. When you are ready to go the other way, you can use rep scasb to look for an item in the table, and subtract the difference between the location of the item in the table and the beginning of the table to get the index of the item.

Hope this helps.

Re: Encoding help, run into block.
 

Thanks~ Definately helps, I was able to make a working program that perfectly encodes into Base64 100% of the time. As for decoding I decided to start working on it soon, thanks for the tip on how to go the other way!